The increasing importance of the Internet of Things (IoT) has driven the development of wearable electronic devices, particularly wearable displays. Textile-based displays offer advantages in various applications due to their compatibility with fabric substrates. Organic light-emitting diodes (OLEDs) are attractive for wearable displays because of their flexibility, lightweight nature, low power consumption, and low heat generation. However, challenges remain in creating durable and washable textile-based OLEDs. The key challenge lies in the encapsulation layer, which protects the water-sensitive OLED from environmental factors. Existing encapsulation methods often involve high temperatures, damaging the OLED's low thermal stability, and lack sufficient flexibility and waterproofness. Previous research using materials like CeO₂ and PDVB showed insufficient WVTR, while Al₂O₃-based barriers lacked flexibility and waterproof properties, or suffered from low throughput and required plasma treatment. This research aims to overcome these limitations by developing a novel multi-functional near-room-temperature encapsulation layer for foldable and washable textile-based OLEDs, addressing both the material limitations and the processing constraints.

The literature review section highlights the existing limitations in fabricating reliable, foldable, and washable textile-based OLEDs. It discusses previous attempts at room-temperature encapsulation, pointing out their deficiencies in WVTR, flexibility, and waterproof properties. Specific examples are given of research using CeO₂ and poly(divinylbenzene) (PDVB), and Al₂O₃ and alucone, highlighting their limitations compared to the desired WVTR of 10⁻⁶ g m⁻² day⁻¹. The lack of sufficient flexibility (bending radius <5 mm) and limited waterproof properties (<200 min) in previously reported encapsulation layers are emphasized. The paper underscores the need for a new approach to encapsulation that addresses these shortcomings.

The researchers fabricated textile-based OLEDs with a novel multi-functional near-room-temperature encapsulation layer. The encapsulation comprised a bilayer structure: a nanolaminate layer of Al₂O₃ and TiO₂ deposited via atomic layer deposition (ALD) at 40°C, followed by a highly cross-linked 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (pV3D3) polymer film deposited via initiated chemical vapor deposition (iCVD). The TiO₂ layer was selected for its superior impermeability at near-room-temperature, which is crucial for maintaining the OLED's stability. The Al₂O₃ layer enhances the transmittance and acts as a crack decoupling agent within the nanolaminate structure. The pV3D3 polymer layer provided hydrophobicity and compressive stress to compensate for the tensile stress in the ALD layers, improving flexibility. The thickness of both the nanolaminate layer and the pV3D3 layer were carefully optimized to minimize WVTR and maximize flexibility and transparency. The OLED structure itself was fabricated using thermal evaporation on a planarized polyester textile substrate. WVTR was measured using an electrical calcium test. Detailed characterizations including TEM, EDS, FTIR, and residual stress measurements were performed to assess the properties of the encapsulation layer and the overall device performance. The mechanical durability and waterproof properties of the encapsulated OLEDs were evaluated through bending tests and water immersion experiments.

The key findings demonstrate the successful fabrication of foldable and washable textile-based OLEDs with significantly improved performance. The near-room-temperature ALD TiO₂ film showed unexpectedly high impermeability, surpassing even Al₂O₃ multi-barrier layers deposited at higher temperatures. The unique behavior of TiO₂ at low temperatures was attributed to a low energy barrier for ligand exchange and the avoidance of precursor desorption that causes increased residual stress at higher temperatures. Careful optimization of the nanolaminate structure (3nm sub-layer thickness, 30nm total thickness) minimized WVTR while maintaining transmittance. The compressive stress of the iCVD-deposited pV3D3 layer effectively compensated for the tensile stress in the ALD nanolaminate layer, resulting in a bilayer structure with near-zero residual stress. This stress compensation significantly enhanced the flexibility (critical strain increased to 2%), allowing for bending with a 1.5 mm radius for 1000 cycles without performance degradation. The hydrophobic pV3D3 layer ensured excellent waterproof properties, with the OLED maintaining its performance after 1440 minutes (24 hours) of water immersion. The encapsulated textile-based OLED exhibited an operating lifetime of 160 hours under ambient conditions (compared to 6 hours for unencapsulated OLEDs), demonstrating its robustness and reliability. The device successfully demonstrated functionality when integrated into common clothing items (dress shirt and T-shirt).

The findings address the research question by demonstrating the feasibility of creating highly durable and washable textile-based OLEDs. The superior performance compared to previous research highlights the effectiveness of the multi-functional encapsulation strategy. The near-room-temperature process is crucial for preventing thermal damage to the organic components, while the stress-compensating bilayer structure significantly improves flexibility. The hydrophobic and chemically stable pV3D3 capping layer provides excellent protection against water damage, making the OLEDs suitable for practical applications in wet environments. The success in integrating the OLED into everyday clothing suggests significant progress towards smart e-textiles.

This study successfully fabricated reliable, foldable, and washable textile-based OLEDs using a novel multi-functional near-room-temperature encapsulation layer. The optimized bilayer encapsulation, featuring TiO₂ and a cross-linked polymer, achieved high impermeability, flexibility, and waterproofness. The results pave the way for the development of advanced smart e-textiles and provide a valuable alternative encapsulation strategy for various flexible electronic devices sensitive to temperature and moisture.

While the study demonstrates significant advancements, limitations exist. The long-term stability of the encapsulation layer under extreme environmental conditions (e.g., prolonged exposure to high humidity or UV radiation) requires further investigation. The current design focuses on red phosphorescent OLEDs; further research is needed to extend the approach to other colors and display technologies. The scalability and cost-effectiveness of the fabrication process are also important factors for future commercialization.